As an injection molding machine, there is known an in-line screw type injection molding machine, which moves a screw backward while rotating the screw to melt a plastic material while metering the plastic material, and then suspends the rotation of the screw and moves the screw forward to inject the molten resin into a mold and fill the mold with the molten resin. This type injection molding machine has a metering motor as means for rotationally driving the screw, and an injection motor as means for injecting the molten resin to fill the mold therewith, and these motors are disposed axially in series (see Patent Document 1).
The metering motor is, for example, coupled with a base coupled with a rear end of the screw, and the injection motor rotationally drives a screw shaft abutting against the base so as to axially move the base along the screw shaft.
Then, the metering motor is driven to rotate the screw, while the injection motor is driven to drive the screw shaft, so as to move the screw backward while rotating the screw to melt the resin while metering the resin and to rotate the screw shaft reversely to thrust the screw forward through the screw shaft to thereby inject the resin.
FIG. 4 is a diagram for explaining a control circuit of an in-line screw type injection molding machine in the background art. The injection molding machine has a screw 6 which is, for example, disposed in a heating cylinder rotatably and movably forward/backward, a metering motor 9 which rotationally drives the screw 6, and an injection motor 16 which drives the screw 6 forward/backward.
When the screw 6 in the heating cylinder is rotated backward (clockwise in view from the illustrated motor shaft end), raw resin (for example, thermoplastic resin) supplied from a hopper is kneaded in the heating cylinder and moved toward the front end (left side) of the screw while being plasticized. Thus, metered molten resin is accumulated on the front end side. Next, the injection motor 16 is rotated forward suddenly. On this occasion, the screw 6 is pressed to the illustrated left so as to move forward suddenly. In this manner, the molten resin is injected into a not-shown mold through a nozzle.
An injection motor encoder 57 measures the rotational position of the injection motor 16, and a metering motor encoder 58 measures the rotational position of the metering motor 9. The injection encoder 57 is an absolute type encoder which outputs the absolute value of the rotational position, and the metering motor encoder 58 is an incremental encoder. A load cell 49 is a sensor for measuring an injection pressure and a back pressure imposed on the screw 6.
In FIG. 4, xij0 designates a backward position command pattern signal indicating a backward position of the screw, vij0 designates a backward speed command pattern signal indicating a backward speed of the screw, bp0 designates aback pressure setting pattern signal for setting a back pressure to be imposed on the screw, and vcg0 designates a metering motor rotational speed setting pattern signal for setting a rotational speed of the metering motor. These signals are, for example, supplied from a not-shown host controller.
A deviation e1 between the backward position command pattern signal xij0 and a screw position signal xijm is taken by an adder 32 using the screw position signal xijm as a feedback signal. The injection motor 16 is feedback-controlled based on the deviation e1.
The screw position signal xijm can be obtained based on the rotational displacement of the injection motor 16 from a reference position.
A PID controller 33 for the backward position command calculates an operation quantity u1 with which the screw position should be operated, based on the deviation e1. A speed calculator 34 calculates a speed command v1 based on the operation quantity u1. An adder 35 adds the backward speed command pattern signal vij0 as a feed-forward signal to the speed command v1 to obtain a backward speed control value v3. A minimum value selector 36 selects a smaller one of a back-pressure speed command calculated value v2, which will be described later, and the backward speed control value v3. The minimum value selector 36 outputs the selected value as a screw backward speed command vij.
A servo amplifier 38 controls the rotation of the injection motor 16 in accordance with the speed command vij. The rotational position of the injection motor is supplied to the adder 32 through the servo amplifier 38.
A PID controller 44 for setting the back pressure calculates an operation quantity u2 based on a deviation e2 between the back pressure bp0 indicated by the backpressure setting pattern and the back pressure measured by the load cell 49, which deviation e2 is obtained by an adder 43. A speed calculator 45 calculates a back-pressure speed command v2 based on the operation quantity u2, and supplies the calculation result to the minimum value selector 36. Thus, even when the back-pressure speed command value v2 is excessively large to pass over the backward position of the screw 6, the screw 6 can be prevented from passing over the position set by the backward position command pattern xij0
The metering motor rotational speed setting pattern signal vcg0 is supplied to a servo amplifier 47. The servo amplifier 47 controls driving of the metering motor 9 in accordance with the metering motor rotational speed setting pattern signal vcg0.
Patent Document 1: JP-A-5-345337